PRESIDENT OBAMA was right to order military action to prevent a potential genocide in northern Iraq and to stop forces of the al-Qaeda-derived Islamic State from advancing on Baghdad or the Kurdish capital of Irbil. However, the steps the president authorized on Thursday amount to more of his administration’s half-measures, narrowly tailored to this week’s emergency and unconnected to any coherent strategy to address the conflagration spreading across the Middle East.

While U.S. airstrikes and drops of supplies may prevent the terrorist forces from massacring the Yazidi sect or toppling the pro-Western regime in Kurdistan, Mr. Obama lacks a plausible plan for addressing the larger threat posed by the Islamic State. In recent weeks, senior U.S. officials have described the danger in hair-curling terms: The Islamic State forces, which have captured large numbers of U.S.-supplied heavy weapons, threaten not only the Iraqi and Kurdish governments, but also Lebanon, Turkey and Jordan. With hundreds of Western recruits, they have the ambition and capability to launch attacks against targets in Europe and the United States.

Yet by the White House’s own account, the measures ordered by Mr. Obama are not intended to defeat the Islamic State or even to stop its bloody advances in most of the region. Instead they are limited to protecting two cities where U.S. personnel are stationed and one mass of refugees. The hundreds of thousands of people in Syria, Lebanon and elsewhere threatened by the al-Qaeda forces will receive no U.S. protection. Nor will the terrorists’ hold over the areas they already control, including the large city of Mosul and nearby oil fields, be tested by U.S. airpower.

U.S. officials say that Mr. Obama has refrained from a broader campaign because he believes the Islamic State is “an Iraqi responsibility,” as Defense Secretary Chuck Hagel put it. The administration is pushing Iraq’s political factions, sharply divided along sectarian lines, to join in forming a new government; once such a government is formed, Mr. Obama said, “the United States will work with it and other countries in the region to provide increased support.”

The White House is hopeful that a new prime minister could be nominated this weekend. Even if that occurs, it probably will take Iraqis many more weeks to agree on a common political program, if they are able to do so at all. Kurds and Sunnis are demanding a major decentralization of power, and one of the “other countries” that the United States must balance is Iran, which seeks to perpetuate Shiite dominance in Baghdad. Meanwhile, as senior Kurdish leaders told the administration in a visit to Washington last month, Iraqi army and Kurdish forces probably cannot defeat the Islamic State on their own.

It’s past time for Mr. Obama to set aside a policy that is both minimalist and unrealistic. The United States should offer sustained military support to friendly forces that fight the Islamic State, beginning with the Kurds and including moderate Syrian rebels and Iraqi Sunni tribesmen. It should seek to erode the Islamic State’s military power as much as possible with airstrikes. It should not press for a new Iraqi government unless Shiite leaders and their Iranian sponsors agree to a fundamental restructuring of power. And it should forge a political and diplomatic strategy that encompasses both Iraq and Syria and their interrelated conflicts. The primary aim should not be to minimize U.S. involvement — as Mr. Obama would have it — but to defeat the forces that are destroying the region.